In recent years, optical fibers have been used as strain sensors for sensing the strain, or stress, placed on a structure. The structure may be, for example, a concrete piling used in a building, a tower, a rotor blade of a windmill, or a wing of an airplane. In such environments, a portion of the strain-sensing fiber is embedded in or attached to the structure. Typically, an adhesive material, such as epoxy, is used to attach the strain-sensing fiber to the structure. The ends of the strain-sensing fiber are optically coupled to measurement equipment. A reference optical fiber is typically laid alongside the strain-sensing fiber on the structure to which the strain-sensing fiber is attached. The ends of the reference fiber are also optically coupled to the measurement equipment.
A laser diode or a light emitting diode (LED) of the measurement equipment is modulated to produce a modulated light beam. An optical splitter of the measurement equipment splits the modulated light beam into first and second modulated light beams, which are then optically coupled into the first ends of the strain-sensing fiber and the reference fiber. The first and second modulated light beams propagate along the two fibers and pass out of the second ends of the fibers. The measurement equipment includes first and second optical sensors that receive the respective light beams and convert the respective light beams into respective electrical signals. Electrical circuitry of the measurement equipment processes the electrical signals to determine the phase differences between them. The phase differences are then used to determine the difference in the lengths of the two fibers.
If stress on the strain-sensing fiber has caused it to become elongated, the measurement equipment will calculate the extent of the elongation over time based on the measured phase differences. The extent of the elongation over time may be used to characterize the strain or stress that has been placed on the structure over time, which, in turn, may be used as a factor in determining the integrity of the structure.
One of the disadvantages of the current approach to measuring the amount of strain that has been placed on the structure is that the measurement equipment is complex, expensive and does not always obtain precise measurements. Every receiver channel uses a separate transimpedance amplifier (TIA) and separate phase detection circuitry to measure the phase of the corresponding received signal. The need to use separate TIAs and phase detection circuitry for each receiver channel increases the system complexity and costs. In addition, the TIAs can introduce phase uncertainty, which can reduce measurement precision.
A need exists for a less complex and more cost effective solution for performing the phase measurements, as well as one that obtains more precise measurements.